I'm diving into my first Apple II restoration job on a machine that is just plain dead. Power on the PSU hums, but no light and no bell (let alone video). I was hoping to get some initial directions and some information on this particular machine. It's a fairly late serial number A2S2 (dating to late 1982) with the rev D RFI motherboard. There are a number of interesting upgrades, including a "lowercase chip", a "Repeaterrr" board, and a few expansion cards (Microsoft RAM, Apple Super Serial, Applied Engineering VideoMaster.)
I've started with the power supply and determined the following:
1. The voltages on the motherboard are all off. With reference to ground on pin 16 of the last memory chip the following voltages are measured:
pin 1 -0.4V
pin 8 1.8V
pin 9 0.69V
So the relative magnitudes and polarities are correct (pin 1 should be -5V, pin 8 should be +12V, pin 9 should be +5V) but the voltages are obviously way off. I confirmed these voltages on the expansion slots as well.
Searching previous discussions here and elsewhere, it is mentioned that this probably indicates a short on the motherboard and not necessarily a problem with the PSU (http://www.vintage-computer.com/vcforum/archive/index.php/t-13620.html)
2. After this, I checked the resistances on the pins relative to ground:
Pin 1: 47.5 Ohm
Pin 8: 51.2 Ohm
Pin 9: 5 Ohm
The above discussion suggests that anything below 10 Ohm indicates a short. So can I conclude that the +5V line is shorted? Is this likely to be on the motherboard or in the PSU?
3. The power supply "hums" (sounds like 60Hz), but I'm not sure it is "chirping". It is somewhat "metallic" sounding. I'm not sure if this is the "clicking" some refer to that could indicate a problem in the low voltage side of the PSU circuit.
4. All internals look ok on both the motherboard and PSU. No obvious blown capacitors and the PSU fuse is good.
A few other things I've noted on this machine.
1. The PSU unit is unlike any I've seen. It is black, has no Apple labeling, and appears to be a different internal design than that described in the SAMS book or red book. The model is "PS-A" and it is made in Taiwan (see attached photo). I'm guessing that this is a replacement PSU? The back has a switch that I assume allows the power supply to be selected between 110 and 240V. There is a plate that is screwed down holding the switch in place and a label that says "110V". So perhaps this was a power supply that could be fitted in either the US or Europlus models?
2. The RAM installed in this machine is not made up of the 4116 chips, but National Semiconductor 5290N-3. The reference sheets I find on these indicate that these are 16K x 1 chips? This does not make sense since they should be 16K x 8.
3. For what its worth, documentation I have for this machine indicates that it was manufactured in Texas. Do the different parts correspond to different manufacturing sites or were all machines assembled in Texas at that point?
Any information would be greatly appreciated!